Lubricating oil composition

ABSTRACT

Provided is a lubricating oil composition containing (A) 70 mass % or more and 95 mass % or less of a poly-α-olefin base oil, (B) 5 mass % or more and 20 mass % or less of an ester-based base oil, and (C) an ester-based defoaming agent, the poly-α-olefin base oil (A) being a polymer of an α-olefin having 8 to 12 carbon atoms. The lubricating oil composition is able to keep excellent defoaming performance over a long period of time and is suitable as a step-up gear oil composition for wind power generation.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition, andparticularly to a lubricating oil composition which is suitable as astep-up gear oil composition for wind power generation.

BACKGROUND ART

Wind power generation, making use of renewable energy, is expected toincrease in demand, from the viewpoints of reduction of the environmentload and the conservation of fossil fuel which have been argued inrecent years.

In wind power generation employing wind force, kinetic energy of wind istransformed into power by means of rotors including propellers which arerotated by wind. By means of the generated power, a power generator isoperated to produce electric energy.

In the wind power generation, a step-up gear is employed to elevate therotation rate of rotors, which is relatively slow, so as to improvepower generation efficiency of the power generator. Actually, a varietyof step-up gears are known in the art, and of those, a planetarygear-type power transmission apparatus is widely employed.

A step-up gear oil composition for wind power generation which is usedfor such a planetary gear-type power transmission apparatus or the likeis required to be maintenance-free and operable over a long period oftime.

As the lubricating oil composition which is used for a step-up gear forwind power generation, PTL 1 discloses those containing a poly-α-olefinand a polyol ester in the working examples.

CITATION LIST Patent Literature

PTL 1: WO 2008/038701 A1

SUMMARY OF INVENTION Technical Problem

However, as a result of extensive and intensive investigations made bythe present inventor, it has been found that if a lubricating oilcomposition containing a poly-α-olefin and an ester-based base oil isused for an application of a step-up gear oil for wind power generation,there is a concern that seizure attributable to foaming is generated.Furthermore, it has been found that even in the case where a defoamingagent is added for the purpose of inhibiting the foaming, the defoamingperformance is significantly deteriorated during a long-term operationdepending upon the kind of a defoaming agent.

Under such circumstances, an object of the present invention is toprovide a lubricating oil composition which is able to keep excellentdefoaming performance over a long period of time and which is suitableas a step-up gear oil composition for wind power generation.

Solution To Problem

In order to develop a lubricating oil composition having theaforementioned excellent performances, the present inventor madeextensive and intensive investigations. As a result, it has been foundthat the aforementioned problem can be solved by a combination of (A) apoly-α-olefin base oil, (B) an ester-based base oil, and (C) anester-based defoaming agent. The present invention has been accomplishedon the basis of such finding.

Specifically, the present invention provides the following.

-   [1] A lubricating oil composition containing (A) 70 mass % or more    and 95 mass % or less of a poly-α-olefin base oil, (B) 5 mass % or    more and 20 mass % or less of an ester-based base oil, and (C) an    ester-based defoaming agent, the poly-α-olefin base oil (A) being a    polymer of an a-olefin having 8 to 12 carbon atoms.-   [2] The lubricating oil composition as described in [1] above,    wherein a sum total of the poly-α-olefin base oil (A) and the    ester-based base oil (B) is 90 mass % or more on the basis of the    total amount of the composition.-   [3] The lubricating oil composition as described in [1] or [2]    above, wherein the ester-based defoaming agent (C) is composed of a    (meth)acrylate polymer having a weight average molecular weight of    10,000 or more and 100,000 or less.-   [4] The lubricating oil composition as described in any of [1] to    [3] above, wherein the poly-α-olefin base oil (A) is a decene    oligomer.-   [5] The lubricating oil composition as described in any of [1] to    [4] above, wherein the ester-based base oil (B) is a polyol ester.-   [6] The lubricating oil composition as described in any of [1] to    [5] above, which is a step-up gear oil composition for wind power    generation.

Advantageous Effects of Invention

According to the present invention, it is possible to provide alubricating oil composition which is able to keep excellent defoamingperformance over a long period of time and which is suitable as astep-up gear oil composition for wind power generation.

DESCRIPTION OF EMBODIMENTS

The lubricating oil composition of the present invention contains (A) 70mass % or more and 95 mass % or less of a poly-α-olefin base oil, (B) 5mass % or more and 20 mass % or less of an ester-based base oil, and (C)an ester-based defoaming agent.

[(A) Poly-α-Olefin Base Oil]

The poly-α-olefin base oil (A) is a polymer obtained by using one ormore α-olefins having 8 to 12 carbon atoms.

The poly-α-olefin base oil (A) is suitably one having a kinematicviscosity at 40° C. in the range of 30 mm²/s or more and 800 mm²/s orless and a viscosity index of 80 or more. When the kinematic viscosityat 40° C. is 30 mm²/s or more, an evaporation loss is small, whereaswhen it is 800 mm²/s or less, a power loss caused by viscous resistancedoes not excessively increase. The kinematic viscosity at 40° C. is morepreferably 32 mm²/s or more and. 680 mm²/s or less, and especiallypreferably 100 mm²/s or more and 500 mm²/s or less. When the viscosityindex is 80 or more, a variation in viscosity caused by a temperaturevariation is small. This viscosity index is more preferably 100 or more,and still more preferably 130 or more.

The poly-α-olefin base oil (A) preferably has a pour point of −25° C. orlower. When the pour point is −25° C. or lower, the lubricating oilcomposition of the present invention containing the same has sufficientfluidity even in a low-temperature environment. The pour point of thepoly-α-olefin base oil (A) is more preferably −30° C. or lower, andstill more preferably −40° C. or lower.

It is to be noted that the aforementioned kinematic viscosity andviscosity index are values as measured in accordance with JIS K2283, andthe pour point is a value as measured in accordance with JIS K2265.

A content of the poly-α-olefin base oil (A) in the lubricating oilcomposition of the present invention is 70 mass % or more and 95 mass %or less, preferably 75 mass % or more and 95 mass % or less, and morepreferably 80 mass % or more and 92 mass % or less on the basis of thetotal amount of the composition. When the content of the poly-α-olefinbase oil (A) is less than 70 mass %, high anti-seizure performance andanti-fatigue performance which are required for the step-up gear oil forwind power generation are not obtained, and defoaming property becomesinsufficient, whereas when it is more than 95 mass %, a content of theester-based base oil (B) becomes insufficient.

[(B) Ester-Based Base Oil]

As the ester-based base oil (B), a polyol ester is preferably used.Though the polyol ester may be either a partial ester of a polyol or acomplete ester of a polyol, the use of a partial ester of a polyol ispreferred from the viewpoint of sludge solubility.

Though the polyol serving as a raw material of the polyol ester is notparticularly limited, it is preferably an aliphatic polyol. Examples ofthe polyol may include dihydric alcohols, such as ethylene glycol,diethylene glycol, triethylene glycol, propylene glycol, trimethyleneglycol, tetramethylene glycol, neopentyl glycol, etc.; trihydricalcohols, such as glycerin, trimethylolethane, trimethylolpropane, etc.;and tetrahydric or polyhydric alcohols, such as diglycerin, triglycerin,pentaerythritol, dipentaerythritol, mannite, sorbit, etc.

The hydrocarbyl group constituting the polyol ester is preferably analkyl group or an alkenyl group each having 6 to 30 carbon atoms, andmore preferably an alkyl group or an alkenyl group each having 12 to 24carbon atoms. Examples thereof may include various hexyl groups, octylgroups, decyl groups, dodecyl groups, tetradecyl groups, hexadecylgroups, heptadecyl groups, octadecyl groups, hexenyl groups, octenylgroups, decenyl groups, dodecenyl groups, tetradecenyl groups,hexadecenyl groups, octadecenyl groups, and the like.

The alkyl group or alkenyl group may be linear or branched.

Specific examples of the complete ester of polyol may include, but arenot limited to, neopentyl glycol dilaurate, neopentyl glycoldimyristate, neopentyl glycol dipalmitate, neopentyl glycol distearate,neopentyl glycol diisostearate, trimethylolpropane trilaurate,trimethylolpropane trimyristate, trimethylolpropane tripalmitate,trimethylolpropane tristearate, trimethylolpropane triisostearate,glycerin trilaurate, glycerin tristearate, glycerin triisostearate, andthe like.

The partial ester of polyol is not particularly limited so long as atleast one hydroxyl group remains.

Specific examples of the partial ester of polyol may include, but arenot limited to, neopentyl glycol monolaurate, neopentyl glycolmonomyristate, neopentyl glycol monopalmitate, neopentyl glycolmonostearate, neopentyl glycol monoisostearate, trimethylolpropane mono-or dilaurate, trimethylolpropane mono- or dimyristate,trimethylolpropane mono- or dipalmitate, trimethylolpropane mono- ordistearate, trimethylolpropane mono- or diisostearate, glycerin mono- ordilaurate, glycerin mono- or distearate, glycerin mono- ordiisostearate, and the like.

The polyol ester is preferably one represented by the following generalformula (B-1) from the viewpoints of foaming prevention property and thelike.

(R¹—COO—)_(n)L  (B-1)

In the formula, R¹ is a hydrocarbon group having 6 to 30 carbon atoms; nis an integer of 2 to 4; and L is an n-valent hydrocarbon group.

R¹ is preferably an alkyl group or an alkenyl group each having 12 to 24carbon atoms.

n is preferably 3 or 4.

L is preferably an n-valent hydrocarbon group having 3 to 10 carbonatoms.

In the lubricating oil composition of the present invention, the polyolester may be used singly or in combination of two or more thereof as theester-based base oil (B). A blending amount thereof is 5 mass % or moreand 20 mass % or less, and preferably 7 mass % or more and 15 mass % orless on the basis of the total amount of the composition. When thecontent of the ester-based base oil is less than 5 mass %, the sludgesolubility becomes insufficient, and the solubility of an additivebecomes insufficient.

In the lubricating oil composition of the present invention, a sum totalof the poly-α-olefin base oil (A) and the ester-based base oil (B) ispreferably 90 mass % or more, more preferably 92 mass % or more and 99mass % or less, and still more preferably 93 mass % or more and 98 mass% or less on the basis of the total amount of the composition.

[Other Base Oil]

The lubricating oil composition of the present invention may containother base oil than the aforementioned poly-α-olefin base oil (A) andester-based base oil (13), and its content is preferably less than 25mass %, more preferably 15 mass % or less, and still more preferably 10mass % or less on the basis of the total amount of the composition.

The other base oil is not particularly limited with respect to its kind,and either a mineral oil or a synthetic oil may be used. A variety ofconventional mineral oils may be used, and examples thereof includeparaffin base mineral oils, intermediate mineral oils, naphthene basemineral oils, and the like. Specific examples thereof includesolvent-refined or hydrogenated light neutral oils, medium neutral oils,heavy neutral oils, and bright stocks, and the like.

A variety of conventional synthetic oils may be used. For example,polybutene, phosphate esters, polyphenyl ethers, alkylbenzenes,alkylnaphthalenes, polyoxyalkylene glycols, neopentyl glycol, siliconeoil, trimethylolpropane, pentaerythritol, hindered esters, and the likemay be used. These base oils may be used singly or in combination of twoor more thereof, and a mineral oil and a synthetic oil may be used incombination.

[(C) Ester-Based Defoaming Agent]

The ester-based defoaming agent (C) is preferably one composed of a(meth)acrylate polymer. It is to be noted that in the presentspecification, the term “(meth)acrylate” means that all of “acrylate”and “methacrylate” are included.

The (meth)acrylate polymer is preferably one having a weight averagemolecular weight of 10,000 or more and 100,000 or less, and morepreferably one having a weight average molecular weight of 20,000 ormore and 80,000 or less.

[(D) Sulfur Compound]

The lubricating oil composition of the present invention may furthercontain (D) a sulfur compound.

Specific examples of the sulfur compound include (d-1) a sulfur compoundcontaining no poly-sulfur bond which is equal to or longer than —S—S—S—in a molecule thereof and having an S content within the molecule of 15mass % or more and (d-2) a thiophosphoric acid trihydrocarbyl esterrepresented by the following general formula (I).

(R—O—)₃P═S  (I)

In the formula, R represents a hydrocarbyl group having 6 to 20 carbonatoms.

Examples of the sulfur compound serving as the component (d-1) mayinclude the following compounds.

(1) Olefin mono- or disulfide

(2) Dihydrocarbyl mono- or disulfide

(3) Thiadiazole compound

(4) Dithiocarbamate compound

(5) Ester compound having a disulfide structure

(6) Other sulfur compound

[Olefin Mono- or Disulfide]

Examples of the olefin mono- or disulfide may include compoundsrepresented by the following general formula (III) and the like.

R¹—S_(a)—R²  (III)

In the formula, R¹ represents an alkenyl group having 2 to 15 carbonatoms; R² represents an alkyl group or an alkenyl group each having 2 to15 carbon atoms; and a represents 1 or 2.

Such a compound is obtained through a reaction between an olefin having2 to 15 carbon atoms or a dimer to tetramer thereof and a sulfidizingagent, such as sulfur, sulfur chloride, etc., and the olefin ispreferably propylene, isobutene, diisobutene, or the like.

[Dihydrocarbyl Mono- or Disulfide]

Examples of the dihydrocarbyl mono- or disulfide include compoundsrepresented by the following general formula (IV).

R³—S_(b)—R⁴  (IV)

In the formula, R³ and R⁴, which may be identical to or different fromeach other, each represent an alkyl group or a cyclic alkyl group eachhaving 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms,an alkylaryl group having 7 to 20 carbon atoms, or an arylalkyl grouphaving 7 to 20 carbon atoms; and b is 1 or 2.

Here, when R³ and R⁴ are each an alkyl group, the compound representedby the foregoing general formula (IV) is called an alkyl sulfide.

Preferred examples of the dihydrocarbyl mono- or disulfide representedby the foregoing general formula (IV) may include dibenzyl mono- ordisulfide, various dinonyl mono- or disulfides, various didodecyl mono-or disulfides, various dibutyl mono- or disulfides, various dioctylmono- or disulfides, diphenyl mono- or disulfide, dicyclohexyl mono- ordisulfide, and the like.

[Thiadiazole Compound]

Preferred examples of the thiadiazole compound may include2,5-bis(n-hexyldithio)-1,3,4-thiadiazole,2,5-bis(n-octyldithio)-1,3,4-thiadiazole,2,5-bis(n-nonyldithio)-1,3,4-thiadiazole,2,5-bis(1,1,3,3-tetramethylbutyldithio)-1,3,4-thiadiazole,3,5-bis(n-hexyldithio)-1,2,4-thiadiazole,3,6-bis(n-octyldithio)-1,2,4-thiadiazole,3,5-bis(n-nonyldithio)-1,2,4-thiadiazole,3,5-bis(1,1,3,3-tetramethylbutyldithio)-1,2,4-thiadiazole,4,5-bis(n-octyldithio)-1,2,3-thiadiazole,4,5-bis(n-nonyldithio)-1,2,3-thiadiazole,4,5-bis(1,1,3,3-tetramethylbutyldithio)-1,2,3-thiadiazole, and the like.

[Dithiocarbamate Compound]

Examples of the dithiocarbamate compound include alkylenebisdialkyldithiocarbamates. Of those, dithiocarbamate compounds havingan alkylene group having 1 to 3 carbon atoms, a linear or branched,saturated or unsaturated alkyl group having 3 to 20 carbon atoms, or acyclic alkyl group having 6 to 20 carbon atoms are preferably used.Specific examples of such dithiocarbamate compounds include methylenebisdibutyldithiocarbamate, methylene bisdioctyldithiocarbamate,methylene bistridecyldithiocarbamate, and the like.

[Ester Compound Having a Disulfide Structure]

As the ester compound having a disulfide structure, a disulfide compoundrepresented by the general formula (V) and a disulfide compoundrepresented by the general formula (VI) are suitably used.

R⁵OOC-A¹-S—S-A²-COOR⁶  (V)

R¹¹OOC—CR¹³R¹⁴—CR¹⁵(COOR¹²)—S—S—R²⁰(COOR¹⁷)—CR¹⁸R¹⁹—COOR¹⁶  (VI)

In the foregoing general formula (V), each of R⁵ and R⁶ independentlyrepresents a hydrocarbyl group having 1 to 30 carbon atoms, preferably ahydrocarbyl group having 1 to 20 carbon atoms, more preferably ahydrocarbyl group having 2 to 18 carbon atoms, and especially preferablya hydrocarbyl group having 3 to 18 carbon atoms. The hydrocarbyl groupmay be linear, branched, or cyclic and may contain an oxygen atom, asulfur atom, or a nitrogen atom. Though R⁵ and R⁶ may be identical to ordifferent from each other, R⁵ and R⁶ are preferably identical to eachother from the standpoint of production.

Each of A¹ and A² is independently a group represented by CR⁷R⁸ orCR⁷R⁸—CR⁹R¹⁰, wherein each of R⁷ to R¹⁰ is independently a hydrogen atomor a hydrocarbyl group having 1 to 20 carbon atoms. The hydrocarbylgroup is preferably a hydrocarbyl group having 1 to 12 carbon atoms, andmore preferably a hydrocarbyl group having 1 to 8 carbon atoms. ThoughA¹ and A² may be identical to or different from each other, A¹ and A²are preferably identical to each other from the standpoint ofproduction.

Meanwhile, in the general formula (VI), each of R¹¹, R¹², R¹⁶, and R¹⁷is independently a hydrocarbyl group having 1 to 30 carbon atoms,preferably a hydrocarbyl group having 1 to 20 carbon atoms, morepreferably a hydrocarbyl group having 2 to 18 carbon atoms, andespecially preferably a hydrocarbyl group having 3 to 18 carbon atoms.The hydrocarbyl group may be linear, branched, or cyclic and may containan oxygen atom, a sulfur atom, or a nitrogen atom. Though R¹¹, R¹², R¹⁶,and R¹⁷ may be identical to or different from each other, R¹¹, R¹², R¹⁶,and R¹⁷ are preferably identical to each other from the standpoint ofproduction.

Each of R¹³ to R¹⁵ and R¹⁸ to R²⁰ is independently a hydrogen atom or ahydrocarbyl group having 1 to 5 carbon atoms. Each of R¹³ to R¹⁵ and R¹⁸to R²⁰ is preferably a hydrogen atom from the standpoint of easiness ofavailability of raw materials.

Specific examples of the disulfide compound represented by the foregoinggeneral formula (V) may include bis(methoxycarbonylmethyl) disulfide,bis(ethoxycarbonylmethyl) disulfide, bis(n-propoxycarbonylmethyl)disulfide, bis(isopropoxycarbonylmethyl) disulfide,bis(cyclopropoxycarbonylmethyl) disulfide,1,1-bis(1-methoxycarbonylethyl) disulfide,1,1-bis(1-methoxycarbonyl-n-propyl) disulfide,1,1-bis(1-methoxycarbonyl-n-butyl) disulfide,1,1-bis(1-methoxycarbonyl-n-hexyl) disulfide,1,1-bis(1-methoxycarbonyl-n-octyl) disulfide,2,2-bis(2-methoxycarbonyl-n-propyl) disulfide,α,α-bis(α-methoxycarbonylbenzyl) disulfide,1,1-bis(2-methoxycarbonylethyl) disulfide,1,1-bis(2-ethoxycarbonylethyl) disulfide,1,1-bis(2-n-propoxycarbonylethyl) disulfide,1,1-bis(2-isopropoxycarbonylethyl) disulfide,1,1-bis(2-cyclopropoxycarbonylethyl) disulfide,1,1-bis(2-methoxycarbonyl-n-propyl) disulfide,1,1-bis(2-methoxycarbonyl-n-butyl) disulfide,1,1-bis(2-methoxycarbonyl-n-hexyl) disulfide,1,1-bis(2-methoxycarbonyl-n-propyl) disulfide,2,2-bis(3-methoxycarbonyl-n-pentyl) disulfide,1,1-bis(2-methoxycarbonyl-1-phenylethyl) disulfide, and the like.

Specific examples of the disulfide compound represented by the foregoinggeneral formula (VI) may include tetramethyl dithiomalate, tetraethyldithiomalate, tetra-1-propyl dithiomalate, tetra-2-propyl dithiomalate,tetra-1-butyl dithiomalate, tetra-2-butyl dithiomalate, tetraisobutyldithiomalate, tetra-1-hexyl dithiomalate, tetra-1-octyl dithiomalate,tetra-1-(2-ethyl)hexyl dithiomalate, tetra-1-(3,5,5-trimethyl)hexyldithiomalate, tetra-1-decyl dithiomalate, tetra-1-dodecyl dithiomalate,tetra-1-hexadecyl dithiomalate, tetra-1-octadecyl dithiomalate,tetrabenzyl dithiomalate, tetra-α-(methyl)benzyl dithiomalate,tetra-α,α-dimethylbenzyl dithiomalate, tetra-1-(2-methoxy)ethyldithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate,tetra-1-(2-butoxy)ethyl dithiomalate, tetra-1-(2-ethoxy)ethyldithiomalate, tetra-1-(2-butoxy-butoxy)ethyl dithiomalate,tetra-1-(2-phenoxy)ethyl dithiomalate, and the like.

[Other Sulfur Compound]

Examples of the other sulfur compound may include sulfidized fats andoils, such as sulfidized lard, sulfidized rape seed oil, sulfidizedcastor oil, sulfidized soy bean oil, sulfidized rice bran oil, etc.;sulfidized fatty acids, such as thioglycolic acid, oleic acid sulfide,etc.; dialkyl thiodipropionate compounds, such as dilaurylthiodipropionate, distearyl thiodipropionate, dimyristylthiodipropionate, etc.; a thioterpene compound formed through a reactionbetween phosphorus pentasulfide and pinene; and the like.

In the lubricating oil composition of the present invention, theaforementioned sulfur compounds may be used singly or in combination oftwo or more thereof as the sulfur compound serving as the component(d-1). As for a blending amount of this component (d-1), the sulfurcompound is contained in an amount of preferably 0.2 mass % or more and0.6 mass % or less, and more preferably 0.3 mass % or more and 0.5 mass% or less in terms of a sulfur atom on the basis of the total amount ofthe lubricating oil composition. When this sulfur atom content is 0.2mass % or more, the anti-seizure performance is improved. On the otherhand, when it is 0.6 mass % or less, not only the anti-fatigueperformance is improved, but also the generation of sludge is inhibited.

If desired, the lubricating oil composition of the present invention maycontain a thiophosphoric acid trihydrocarbyl ester represented by thegeneral formula (I) as the component (d-2).

(R—O—)₃P═S  (I)

In the general formula (I), R represents a hydrocarbyl group having 6 to20 carbon atoms. Examples of this hydrocarbyl group include a linear,branched, or cyclic alkyl group or alkenyl group having 6 to 20 carbonatoms, an aryl group having 6 to 20 carbon atoms, and an aralkyl grouphaving 7 to 20 carbon atoms. In the aryl group and aralkyl group, one ormore alkyl groups may be introduced onto the aromatic ring. Three R—O—groups may be identical to or different from each other.

Examples of the alkyl group and the alkenyl group each having 6 to 20carbon atoms include various hexyl groups, various octyl groups, variousdecyl groups, various dodecyl groups, various tetradecyl groups, varioushexadecyl groups, various octadecyl groups, a cyclohexyl group, varioushexenyl groups, various octenyl groups, various decenyl groups, variousdodecenyl groups, various tetradecenyl groups, various hexadecenylgroups, various octadecenyl groups, a cyclohexenyl group, and the like.

Examples of the aryl group having 6 to 20 carbon atoms include a phenylgroup, a tolyl group, a xylyl group, a decylphenyl group, a2,4-didecylphenyl group, a naphthyl group, and the like. Examples of thearalkyl group having 7 to 20 carbon atoms include a benzyl group, aphenethyl group, a naphthylmethyl group, a methylbenzyl group, amethylphenethyl group, a methylnaphthylmethyl group, and the like.

Specific examples of the thiophosphoric acid trihydrocarbyl esterrepresented by the foregoing general formula (I) include trihexylthiophosphate, tri-2-ethylhexyl thiophosphate, tris(decyl)thiophosphate, trilauryl thiophosphate, trimyristyl thiophosphate,tripalmityl thiophosphate, tristearyl thiophosphate, trioleylthiophosphate, tricresyl thiophosphate, trixylyl thiophosphate,tris(decylphenyl) thiophosphate, tris[2,4-isoalkyl(C₉,C₁₀)phenyl]thiophosphate, and the like. These thiophosphoric acidtrihydrocarbyl phosphates may be used singly or in combination of two ormore thereof.

In the lubricating oil composition of the present invention, thethiophosphoric acid trihydrocarbyl ester serving as the component (d-2)is optionally added for the purpose of further enhancing the additioneffect of the sulfur compound serving as the component (d-1). Itsaddition amount is preferably in the range of 0.1 mass % or more and 1.0mass % or less, and more preferably 0.2 mass % or more and 0.5 mass % orless on the basis of the total amount of the lubricating oilcomposition. A content of the whole of the sulfur atoms in thelubricating oil composition is preferably 150 ppm by mass or more and500 ppm by mass or less.

[(E) Acidic Phosphate Ester Amine Salt]

It is preferred that the lubricating oil composition of the presentinvention further contains (E) an acidic phosphate ester amine salt.

As the acidic phosphate ester amine salt (E), a compound obtained byallowing (e-1) an acidic alkyl phosphate ester in which an alkyl groupthereof has 6 to 20 carbon atoms and (e-2) a dialkylamine and/or atrialkylamine to react with each other in advance may be blended. Byblending such a compound in the lubricating oil composition of thepresent invention, the acidic phosphate ester amine salt (E) may also becontained.

[(e-1) Acidic Alkyl Phosphate Ester]

As the acidic alkyl phosphate ester (e-1), a compound represented by thefollowing general formula (II) is preferred.

In the general formula (II), X¹ represents a hydrogen atom or an alkylgroup having 6 to 20 carbon atoms; and X² represents an alkyl grouphaving 6 to 20 carbon atoms.

The alkyl group having 6 to 20 carbon atoms may be linear, branched, orcyclic. Examples thereof include various hexyl groups, octyl groups,decyl groups, dodecyl groups, tetradecyl groups, hexadecyl groups,octadecyl groups, eicosyl groups, and the like. Of those, alkyl groupshaving 10 to 18 carbon atoms are preferred.

Examples of the acidic alkyl phosphate ester represented by theforegoing general formula (II) may include acidic phosphate monoesters,such as monooctyl acid phosphate, monodecyl acid phosphate, monoisodecylacid phosphate, monolauryl acid phosphate, mono(tridecyl) acidphosphate, monomyristyl acid phosphate, monopalmityl acid phosphate,monostearyl acid phosphate, etc.; and acidic phosphate diesters, such asdioctyl acid phosphate, didecyl acid phosphate, diisodecyl acidphosphate, dilauryl acid phosphate, di(tridecyl) acid phosphate,dipalmityl acid phosphate, distearyl acid phosphate, etc.

In the present invention, the aforementioned acidic phosphate esterserving as the component (e-1) may be used singly or in combination oftwo or more thereof. A blending amount thereof is preferably 150 ppm bymass or more and 500 ppm by mass or less in terms of a content of thewhole of the phosphorus atom in the lubricating oil composition. Whenthe content of the phosphorus atoms is 150 ppm by mass or more, theanti-seizure performance is improved, whereas when it is 500 ppm by massor less, the anti-fatigue performance is improved. The content of thephosphorus atom is more preferably 250 ppm by mass or more and 450 ppmby mass or less, and still more preferably 350 ppm by mass or more and400 ppm by mass or less.

[(e-2) Dialkylamine and/or Trialkylamine]

The dialkylamine and/or the trialkylamine which is used as the component(e-2) is preferably one in which an alkyl group thereof has 6 to 20carbon atoms.

Examples of the dialkylamine may include dihexylamine,dicyclohexylamine, dioctylamine, dilaurylamine, distearylamine, and thelike. Examples of the trialkylamine may include trihexylamine,tricyclohexylamine, trioctylamine, trilaurylamine, tristearylamine, andthe like.

In the lubricating oil composition of the present invention, theaforementioned dialkylamine and/or trialkylamine serving as thecomponent (e-2) may be used singly or in combination of two or morethereof. Of those, trialkylamines are preferred from the standpoint ofperformances. In the case of using a monoalkylamine, the anti-seizureperformance is impaired.

In the lubricating oil composition of the present invention, so long asthe object of the present invention is not impaired, various additives,for example, at least one selected from an ashless detergent-dispersant,an antioxidant, a rust inhibitor, a metal deactivator, a pour pointdepressant, and the like, may be blended, if desired.

Examples of the ashless detergent-dispersant include succinimides,boron-containing succinimides, benzylamines, boron-containingbenzylamines, succinate esters, amides of a mono- or divalent carboxylicacid represented by a fatty acid or succinic acid, and the like.

An amine-based antioxidant, a phenol-based antioxidant, or asulfur-containing antioxidant, all of which are conventionally used inlubricating oils, may be used as the antioxidant. These antioxidants maybe used singly or in combination of two or more thereof. Examples of theamine-based antioxidant include monoalkyldiphenylamine-based compounds,such as monooctyldiphenylamine, monononyldiphenylamine, etc.;dialkyldiphenylamine-based compounds, such as 4,4′-dibutyldiphenylamine,4,4′-dipentyldiphenylamine, 4,4′-dihexyldiphenylamine,4,4′-diheptyldiphenylamine, 4,4′-dioctyldiphenylamine,4,4′-dinonyldiphenylamine, etc.; polyalkyldiphenylamine-based compounds,such as tetrabutyldiphenylamine, tetrahexyldiphenylamine,tetraoctyldiphenylamine, tetranonyldiphenylamine, etc.; andnaphthylamine-based compounds, such as α-naphthylamine;phenyl-α-naphthylamine, butylphenyl-α-naphthylamine,pentylphenyl-α-naphthylamine, hexylphenyl-α-naphthylamine,heptylphenyl-α-naphthylamine; octylphenyl-α-naphthylamine,nonylphenyl-α-naphthylamine, etc.

Examples of the phenol-based antioxidant include monophenol-basedcompounds, such as 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl4-ethylphenol, octadecyl3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, etc.; anddiphenol-based compounds, such as4,4′-methylenebis(2,6-di-tert-butylphenol),2,2′-methylenebis(4-ethyl-6-tert-butylphenol), etc.

Examples of the sulfur-based antioxidant include2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol;thioterpene-based compounds, such as a reaction product betweenphosphorus pentasulfide and pinene; dialkyl thiodipropionates, such asdilauryl thiodipropionate, distearyl thiodipropionate, etc.; and thelike.

Examples of the rust inhibitor may include metal-based sulfonates,succinic acid esters, and the like, and examples of the metaldeactivator may include benzotriazole, thiadiazole, and the like.

As the pour point depressant, polymethacrylates having a weight averagemolecular weight of about 50,000 or more and 150,000 or less and thelike may be used.

The lubricating oil composition of the present invention is excellent inanti-seizure performance and anti-fatigue performance (e.g., FZGmicropitting resistance, etc.) and is reduced in the formation of sludgein an oxidation test. Thus, the lubricating oil composition of thepresent invention can be used as various lubricating oils, for example,gear oil, bearing oil, or the like, and in particular, it is suitable asa lubricating oil for use in a planetary gear-type power transmissionapparatus in a wind power generator or the like.

EXAMPLES

The present invention is hereunder described in more detail by referenceto the following Examples, but it should be construed that the presentinvention is not limited at all by these Examples.

Properties of the base oils employed in the respective Examples andvarious properties of the lubricating oil compositions obtained in therespective Examples were determined in accordance with the followingmethods.

<Properties of Base Oil> (1) Kinematic Viscosity and Viscosity Index

Determined in accordance with JIS K2283.

(2) Foaming Test

300 mL of the lubricating oil composition was taken into a test tube forturbine oil oxidation stability test as prescribed in JIS K2839, and thevolume of foams in a state where air was brown at a rate of 3 L/h via adiffuser as prescribed in JIS K2518 was measured.

A kind of each of the components used in the preparation of thelubricating oil composition is as follows.

-   -   (A) Poly-α-olefin base oil 1 (PAO base oil 1): Decene oligomer,        kinematic viscosity at 40° C.=390 mm²/s, viscosity index=149,        pour point=−50° C. or lower    -   Poly-α-olefin base oil 2 (PAO base oil 2): Decene oligomer,        kinematic viscosity at 40° C.=46 mm²/s, viscosity index=137,        pour point=−50° C. or lower    -   Thickener 1: Ethylene-propylene oligomer, weight average        molecular weight (Mw)=14,400    -   Thickener 2: Polybutene    -   Thickener 3: Hydrogenated petroleum resin    -   (B) Ester-based base oil: Ester of trimethylolpropane and        isostearic acid (molar ratio: 1/2)    -   (C) Ester-based defoaming agent: Defoaming agent component=30 to        40 mass % (weight average molecular weight=50,000), mineral        oil=60 to 70 mass %    -   Silicone-based defoaming agent: Refractive index at 20°        C.=1.405, flash point (COC)=370° C.

(D) Sulfur Compound

-   -   (d-1) Sulfur compound: Methylene bis(dibutyldithiocarbamate)    -   (d-2) Sulfur compound: Tris(isoalkylphenol) thiophosphate        (“IRGALUBE 211”, available from BASF SE)

(E) Acidic Phosphate Ester Amine Salt

-   -   (e-1) Acidic alkyl phosphate ester: Acidic (mono, di)isodecyl        phosphate ester (mixing molar ratio of monoester to diester=1/1)    -   (e-2) Amine: Tri-n-octylamine

Example 1 and Comparative Examples 1 to 4

Lubricating oil compositions having a blending composition shown inTable 1 were prepared, and foaming characteristics of the compositionswere determined. The results are shown in Table 1.

TABLE 1 Exam- Comparative Example ple 1 1 2 3 4 Blending composition(mass %) (A) PAO base oil 1 86.29 86.29  67.29 PAO base oil 2  61.2961.29 Thickener 1  15.00 15.00 Thickener 2  19.00 Thickener 3  10.0010.00 (B) Ester-based base oil 10.00 10.00  10.00 10.00  10.00 (C)Ester-based defoaming 0.01 0.01 agent Silicone-based defoaming 0.01 0.01  0.01 agent (d-1) Sulfur compound 1.65 1.65  1.65 1.65  1.65 (d-2)Sulfur compound 0.40 0.40  0.40 0.40  0.40 (e-1) Acidic alkyl phos- 0.220.22  0.22 0.22  0.22 phate ester (e-2) Amine 0.36 0.36  0.36 0.36  0.36Foaming test Before filtration with Millipore filter After continuousfoaming 20 20  20 20  20 for 1 hour (mL) After continuous foaming 20 70250< 120 250< for 10 hours (mL) After filtration with Millipore filterAfter continuous foaming 20 70 250< 20 250< for 1 hour (mL) Aftercontinuous foaming 20 170 250< 120 250< for 10 hours (mL)

INDUSTRIAL APPLICABILITY

The lubricating oil composition of the present invention is able to keepexcellent defoaming performance over a long period of time, and hence,it is especially suitable as a lubricating oil used for a step-up gearof a planetary gear-type power transmission apparatus or the like to beused in a wind power generator.

1. A lubricating oil composition, comprising (A) 70 mass % or more and95 mass % or less of a poly-α-olefin base oil, (B) 5 mass % or more and20 mass % or less of an ester-based base oil, and (C) an ester-baseddefoaming agent, wherein the poly-α-olefin base oil (A) is a polymer ofan α-olefin comprising 8 to 12 carbon atoms.
 2. The lubricating oilcomposition according to claim 1, wherein a sum total of thepoly-α-olefin base oil (A) and the ester-based base oil (B) is 90 mass %or more based on a total amount of the composition.
 3. The lubricatingoil composition according to claim 1, wherein the ester-based defoamingagent (C) comprises a (meth)acrylate polymer having a weight averagemolecular weight of 10,000 or more and 100,000 or less.
 4. Thelubricating oil composition according to claim 1, wherein thepoly-α-olefin base oil (A) is a decene oligomer.
 5. The lubricating oilcomposition according to claim 1, wherein the ester-based base oil (B)is a polyol ester.
 6. The lubricating oil composition according to claim1, which is a step-up gear oil composition for wind power generation.